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On the exposure of hemispherical photographs in forests
Author
Philip Beckschäfer, Dominik Seidel, Christoph Kleinn and Xu Jianchu
Year
2013
Journal Title
iForest – Biogeosciences and Forestry
Institution
iForest – Biogeosciences and Forestry
Volume
6
Pages
228-237
Call Number
JA0542-14
Keywords
Gap Fraction, Canopy Openness, LAI, Canopy Photography
Abstract:
At least 10 different methods to determine exposure for hemispherical photographs
were used by scientists in the last two decades, severely hampering
comparability among studies. Here, an overview of the applied methods is reported.
For the standardization of photographic exposure, a time-consuming
reference measurement in the open land towards the unobstructed sky was
required so far. The two Histogram Methods proposed here make use of the
technical advances of digital cameras which enable users to assess a photograph’s
histogram directly at the location of measurement. This avoids errors
occurring due to variations in sky lighting happening in the time span between
taking the reference measurement and reaching the sample location within the
forest. The Histogram Methods speed up and simplify taking hemispherical photographs,
and introduce an objectively applicable, standardized approach. We
highlight the importance of correct exposure by quantifying the overestimation
of gap fraction resulting from auto-exposed photographs under a wide range of
canopy openness situations. In our study, gap fraction derived from auto-exposed
photographs reached values up to 900% higher than those derived from
non-overexposed photographs. By investigating the size of the largest gap per
photograph and the number of small gaps (gaps contributing less than 0.1% to
gap fraction), we concluded that the overestimation of gap fraction resulted
mainly from the overexposure of vegetation surrounding large gaps.
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